(1) Field of Invention
This invention relates to a novel extrusion coating composition comprised of a linear low density ethylene hydrocarbon copolymer and a high pressure low density polyethylene homopolymer and/or copolymer and a process for extrusion coating of a substrate or article.
(2) Description of the Prior Art
Long chain branched (LCB), low density polyethylene homopolymers and/or copolymers are polymerized in heavy walled autoclaves or tubular reactors at pressures greater than 15,000 psi and as high as 50,000 psi and at temperatures up to 300.degree. C. As used herein, a "LCB, low density polyethylene homopolymer and/or copolymer" refers to a high pressure, low density polyethylene homopolymer and/or copolymer (also herein referred to as a "high pressure, low density polyethylene"). The molecular structure of high pressure, low density polyethylene is highly complex. The permutations in the arrangement of its simple building blocks are essentially infinite. High pressure, low density polyethylene resins are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of the resins. High pressure, low density polyethylene resins also possess a spectrum of short chain branches, generally 1 to 8 carbon atoms in length, which control resin crystallinity (density). The frequency distribution of these short chain branches is such that, on the average, most chains possess the same average number of branches. The short chain branching distribution characterizing high pressure, low density polyethylene can be considered narrow.
Low density ethylene hydrocarbon copolymers can be produced at low to medium pressures by copolymerizing ethylene with various alpha-olefins using heterogeneous catalysts based on transition metal compounds of variable valence. These low pressure, low density ethylene hydrocarbon copolymer resins are referred to herein as linear, low density ethylene hydrocarbon copolymer resins. These resins generally possess little, if any, long chain branching and the only branching to speak of is short chain branching. Branch length is controlled by comonomer type. Branch frequency is controlled by the concentration of comonomer(s) used during copolymerization. Branch frequency distribution is influenced by the nature of the transition metal catalyst used during the copolymerization process. The short chain branching distribution characterizing transition metal catalyzed low density polyethylene can be very broad.
Low density polyethylene homopolymer and/or copolymer, in general, can exhibit a multitude of properties. It is flexible and has a good balance of mechanical properties such as tensile strength, impact resistance, burst strength, and tear strength. In addition, it retains its strength down to relatively low temperatures. Certain resins do not embrittle at temperatures as low as -70.degree. C. Low density polyethylene homopolymer and/or copolymer, in general, has good chemical resistance. It is relatively inert to acids, alkalis, and inorganic solutions. It is, however, sensitive to hydrocarbons, halogenated hydrocarbons, and to oils and greases. Linear, low density polyethylene homopolymer and/or copolymer, in general, has excellent dielectric strength.
Heretofore, linear, low density ethylene hydrocarbon copolymers have not been commercially employed as extrusion coating compositions. However, high pressure, low density polyethylene is employed for extrusion coating of substrates such as aluminum, paper, polypropylene, polyester, etc. One example of a high pressure, low density polyethylene resin typically used in extrusion coating is a stirred autoclave reactor resin, since it provides, in general, higher extrusion coating rates than can be achieved with tubular reactor resins.